Synchronous generators usually operate together (or in parallel), forming a Synchronous generator converts mechanical power to ac electric power. The. In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then turned by external means producing a. The synchronous electrical generator (also called alternator) belongs to the current (dc) motor or generator, the induction motor or generator, and a number.
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Generally a synchronous generator must have at least 2 components: a) Rotor Windings or Field Windings a. Salient Pole b. Non Salient Pole b) Stator Windings. Synchronous generators or alternators are synchronous machines that convert . of a complete large synchronous generator with a salient-pole rotor with eight. PDF | High-speed synchronous generators rated in the power range of megawatts have been discussed. These generators are nowadays necessary for new.
The DC output from the rectifiers is fed directly into the field of the synchronous generator Fig.
The armature of the ac exciter and the rectifiers are mounted on the main shaft and turn together with the synchronous generator. In comparing the excitation system of Fig. In other words, the commutator which is really a mechanical rectifier is replaced by an electronic rectifier. The result is that the brushes and slip-rings are no longer needed.
The DC control current I c from the pilot exciter regulates the main exciter output I x , as in the case of a conventional DC exciter. The frequency of the main exciter is generally two to three times.
The increase in frequency is obtained by using more poles on the exciter than on the synchronous generator. Static exciters that involve no rotating parts at all are also employed. In this regard, the size of the generator is particularly important because its efficiency automatically improves as the power increases. Another advantage of large machines is that the power output per kilogram increases as the power increases.
From a power standpoint, large machines weigh relatively less than small machines; consequently, they are cheaper. Section Everything, therefore, favors the large machines. However, as they increase in size, we run into serious cooling problems. To prevent an unacceptable temperature rise, we must design efficient cooling systems that become ever more elaborate as the power increases.
For example, a circulating cold air system is adequate to cool synchronous generators whose rating is below 50 MW but between 50 MW and MW we have to resort to hydrogen cooling. Very big generators in the MW range have to be equipped with hollow, water-cooled conductors.
Ultimately, a point is reached where the increased cost of cooling exceeds the savings made elsewhere, and this fixes the upper limit to size.
To sum up, the evolution of big alternators has mainly been determined by the evolution of sophisticated cooling techniques. Other technological breakthroughs, such as better materials, and novel windings have also played a major part in modifying the design of early machines. As regards speed, low-speed generators are always bigger than high-speed machines of equal power.
It is driven at constant speed by a turbine. The leads from the 3-phase, wye-connected stator are brought out to terminals A, B, C, N, and a variable exciting current IX produces the flux in the air gap. Synchronous Generators Let us gradually increase the exciting current while observing the ac voltage Eo between terminal A, say, and the neutral N.
For small values of IX, the voltage increases in direct proportion to the exciting current. However, as the iron begins to saturate, the voltage rises much less for the same increase in IX. If we plot the curve of Eo versus IX, we obtain the no-load saturation curve of the synchronous generator. Up to about 9 kV, the voltage increases in proportion to the current, but then the iron begins to saturate.
Thus, an exciting current of A produces an output of 12 kV, but if the current is doubled, the voltage rises only to 15 kV. The generator is driven by a turbine, and is excited by a DC current IX. The machine and its load are both connected in wye, yielding the circuit of Fig. Although neutrals N1 and N2 are not connected, they are at the same potential because the load is balanced. As affecting the behavior of the voltages or currents in the circuit.
Each phase of the stator winding possesses a resistance R and a certain inductance L. Because this Fig. The impedance is there, but it can neither be seen nor touched. The value of X S is typically 10 to times greater than R; consequently, we can always neglect the resistance, unless we are interested in efficiency or heating effects.
We can simplify the schematic diagram of Fig. In effect, the two other phases are identical, except that their respective voltages and currents are out of phase by degrees.
Furthermore, if we neglect the resistance of the windings, we obtain the very simple circuit of Fig. A synchronous generator can therefore be represented by an equivalent circuit composed of an induced voltage Eo in series with a reactance X S. Synchronous Generators Fig. For a given synchronous reactance, the voltage E at the terminals of the generator depends upon Eo and the load Z. Note that Eo and E are line-to-neutral voltages and I is the line current.
The ac terminals are then short-circuited, and the three line currents are found to be A. Calculate the synchronous reactance per phase.
The equivalent circuit per phase is shown in Fig. The impedance of the circuit is: See Example 5. Actual line voltages and currents. There are many types of loads, but they can all be reduced to two basic categories: In order to construct the phasor diagram for this circuit, we list the following facts: Voltage EX across the synchronous reactance leads current I by 90 degrees.
Both Eo and EX are voltages that exist inside the synchronous generator windings and cannot be measured. The resulting phasor diagram is given in Fig. Furthermore, the internally-generated voltage Eo is greater than the terminal voltage, as we would expect.
What effect does this In some cases the load is somewhat capacitive, so that current I leads the terminal voltage by an angle have on the phasor diagram? The answer is found in Fig.
The voltage E X across the synchronous reactance is still 90 degrees ahead of the current. However, the terminal voltage is now greater than the induced voltage EO , which is a very surprising result. Although it may appear we are getting something for nothing, the higher terminal voltage does not yield any more power. If the load is entirely capacitive, a very high terminal voltage can be produced with a small exciting current.
However, in later chapters, we will see that such under-excitation is undesirable. The no-load saturation curve giving the relationship between Eo and I, is given in Fig. If the excitation is adjusted so that the terminal. No-load b.
Resistive load of 36 MW c. Capacitive load of 12 Mvar Solution: We shall immediately simplify the circuit to show only one phase. The voltage across X S is: With a capacitive load of 12 Mvar: The phasor diagram for this capacitive load is given in Fig.
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Principles of Electrical Machines by V. By Suresh Muthusamy. Download pdf. Synchronous Generator The rotating and stationary parts of an electrical machine can be called as rotor and stator respectively. The rotor or stator of electrical machines acts as a power-producing component and is called as an armature.
The electromagnets or permanent magnets mounted on the stator or rotor are used to provide magnetic field of an electrical machine. The generator in which permanent magnet is used instead of coil to provide excitation field is termed as permanent magnet synchronous generator or also simply called as synchronous generator. Construction of Synchronous Generator In general, synchronous generator consists of two parts rotor and stator. The rotor part consists of field poles and stator part consists of armature conductors.
The rotation of field poles in the presence of armature conductors induces an alternating voltage which results in electrical power generation. Synchronous Generator Working Principle The principle of operation of synchronous generator is electromagnetic induction. If there exits a relative motion between the flux and conductors, then an emf is induced in the conductors.
To understand the synchronous generator working principle, let us consider two opposite magnetic poles in between them a rectangular coil or turn is placed as shown in the below figure. Rectangular Conductor placed in between two opposite Magnetic Poles If the rectangular turn rotates in clockwise direction against axis a-b as shown in the below figure, then after completing 90 degrees rotation the conductor sides AB and CD comes in front of the S-pole and N-pole respectively.
Thus, now we can say that the conductor tangential motion is perpendicular to magnetic flux lines from north to south pole. Thus, we can say that current will pass from A to B and from C to D. If the conductor is rotated in a clockwise direction for another 90 degrees, then it will come to a vertical position as shown in the below figure. Direction of Rotation of Conductor parallel to Magnetic Flux Now, the position of conductor and magnetic flux lines are parallel to each other and thus, no flux is cutting and no current will be induced in the conductor.